When it comes to errors and varieties by the U.S. Mint, there are two important considerations: how does the variation deviate from the original design and how did the variation happen?
Let’s dispense with the first question. Quarters produced in Denver should have a D mintmark and ones in San Francisco, the S mintmark. The Denver quarter should not have a D over S mintmark and the San Francisco quarter, S over D.
Many coin collectors believe that errors are the result of a glitch in the strike process, such as a die cud or chip. But sometimes human error is the cause.
In the case of the 1950 over mintmarks, the first human error occurred at the Philadelphia Mint. Up until 1994, the Philly facility manufactured all the dies of the branch mints. The Denver mint had asked for a certain number of dies, and the fulfillment was one die short. To complete the order, a mint employee in Denver decided to overpunch the die on an existing one, creating the variation. Later in the year the same thing happened at San Francisco, creating the other variety.
You can read more about this on the PCGS CoinFacts website.
Collectors quickly identified the error in 1950 and so kept many over mintmark examples. This means you still can buy uncirculated examples, usually in low mint state condition. More likely, if you are a roll collector, you will find these over mintmarks in worn conditions.
At EF 45, the D over S is worth $150. Retail prices increase to $265 at AU58 and then rise dramatically in uncirculated condition. A gem MS65 example sells for $3,260.
At EF 45, the S over D is worth $210. Retail prices increase to $265 at AU58 and then also rise in uncirculated condition. A gem MS65 example sells for $1,450.
If you are roll searching, you may get lucky and find one of these over mintmarks. But if you are planning to purchase one, buy ones holdered by PCGS or NGC.
Here’s a raw VF30 example from eBay. The photo doesn’t show the mintmark sufficiently to make a determination.
Instead of taking a chance on a raw coin like this, you can find many holdered examples for reasonable prices.
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